Sodium sulfite has a variety of commercial uses such as a disinfectant or bleaching (decoloring) agent for fabrics and paper. It is also used as a preservative in food.
Sodium sulfite (Na2SO3) can be manufactured in a number of ways. Commonly, sodium sulfite is crystallized from a solution of sodium sulfite. Sodium sulfite can be prepared by reacting sulfur dioxide (SO2) in an aqueous, alkaline solution, such as solutions of sodium hydroxide, also known as caustic soda (NaOH), or sodium carbonate (Na2CO3). Sodium sulfite can react with sulfur dioxide to produce sodium bisulfite. When sodium carbonate is used as a reactant, carbonic acid (H2CO3) is a by-product of the reaction. The carbonic acid evolves as carbon dioxide (CO2). The sodium sulfite liquor formed by reaction is pumped to an evaporative crystallizer, where the sodium sulfite is crystallized. These crystals are removed from the crystallizer and dried in a rotary dryer.
A variety of processes have been disclosed for producing sodium sulfite.
U.S. Pat. No. 2,245,697: entitled “Manufacture of alkali metal sulfites,” discloses a process for making sodium sulfite and teaches that water vapor and air introduced into the reactor as diluent in the SO2 feed is vented out along with CO2 that is present in appreciable amounts under certain acidic reactor conditions. The patent also teaches that the liquor is “gassed” with SO2 feed so that all of the CO2 is “expelled.”
U.S. Pat. No. 2,080,528: entitled “Process of manufacturing anhydrous sodium sulfite,” discloses reacting sodium carbonate with sulfur dioxide to make a sodium sulfite liquor that is then boiled to remove residual CO2 gas.
U.S. Pat. No. 2,719,075: entitled “Purification of alkali metal sulfite liquors.” discloses introducing air into a sodium bisulfite liquor to remove CO2.
U.S. Pat. No. 1,937,944: entitled “Manufacture of sulphites,” discloses a process for manufacturing sodium sulphite from sodium carbonate and sulfur dioxide and teaches that the reactor liquor is circulated in the absorbing tower until all the CO2 has “passed off.”
Great Britain Patent No. GB485215: entitled “Process of manufacturing anhydrous sodium sulfite,” teaches that, in a sodium sulfite manufacturing process, caustic soda is added prior to crystallization to remove iron impurities.
France Patent No. FR2534571: entitled “Process and device for production of sodium and potassium sulfite,” teaches the use of heat transfer equipment in the consecutive preparation of Na2SO3, NaHSO3, and Na2S2O5.
Even with the teachings of these patents concerning the various sodium sulfite processes, these disclosures do not recognize nor address the operational and maintenance issues that may occur in heaters associated with commercial-scale sodium sulfite crystallizers. Indeed, fouling of the crystallizer heater can occur so frequently that periodic cleanouts of the system, e.g., use of hydroblasting services, are necessary. Applicants have observed that fouling is especially increased with the use of sodium carbonate as a reactant compared to caustic soda.
Thus, there is a need for process modifications to reduce equipment fouling in a sodium sulfite manufacturing process.
It would be advantageous to modify a process for manufacturing a crystalline alkali product from a liquor, in which a liquor component (e.g., a reactant, an end product, and/or a by-product) may be liberated in gaseous form from the liquor during crystallization, to reduce the operational costs of downtime, increase production rates and reduce the frequency of cleaning out fouled process equipment.